In cross-flow filtration, a feed liquor to be filtered flows through an overflow channel, whereby the feed is directed against the surface of the filter element tangentially. The filter splits the feed into a concentrate (retentate) and a filtrate (permeate). The filter element is generally a polymeric microporous membrane with pores sized so as to be capable of ultrafiltration or microfiltration. Average pore sizes for an ultrafiltration membrane, typically characterized by an exclusion threshold, make possible the retention of macromolecules ranging from 500 to 1,000,000 Daltons, while those for a microfiltration membrane are in the range of 0.01 to 10 μm. The subjects of exclusion thresholds and average pore sizes and their determination are discussed in Scheuermnann, Handbook of Industrial Solids/Liquids Filtration (1990) at pages 250-262.
The retentate is diverted onto the overflow surface of the filter membrane and may be recycled to flow over the same surface again, thus providing for repeated permeate passes. The permeate permeates the filter membrane in a direction substantially perpendicular thereto and is collected below the filter element in a permeate channel and is conducted away from the filtration device. The target substance can be in the permeate and/or in the retentate. Cross-flow filtration devices may have one or more overflow channels. See DE A1 196 36 006 and DE PS 34 41 249. Devices with a multiplicity of overflow channels are mainly in the form of filter cassettes, as disclosed in DE PS 34 41 249. The cassettes consist of a plurality of adjacent filter cells, which, as a rule, are constructed from alternatingly positioned, flat sections of a passage forming an overflow channel for the feed, a first membrane array, a permeate holder for the formation of a filtrate channel, and a second membrane array. Each overflow opening is in fluid communication with an inlet for the feed and with an outlet for the retentate. Each permeate channel is in fluid communication with a permeate outlet.
As retentate is captured it begins to build up and tends to obstruct the pores of the filter element, but by virtue of its tangential flow over the filter's surface, is flushed from the surface, so that the filter's pores are freed for permeation of the feed. In spite of this, for various reasons a layer builds up on the surface of the filter element. Because of this, as a rule, the filtration capacity as well as the operational life of cross-flow filtration devices are diminished.
It is therefore a primary goal of the invention to provide a cross-flow filtration device that avoids the foregoing problem and that exhibits an improved filtration capacity and a substantial operational life.